Severe pipeline damage could result from pressure surges in a liquid pipeline, which could occur when there is any sudden change in velocity of the liquid, as when starting or stopping a pump. Such surges have been relieved in some instances by operation of valve which open whenever they occur. Since many pipeline surges never reach dangerous levels, the diversion of liquid at every occurence is a needless waste and may require a reservoir of enormous capacity. Particularly where a closed tank is required, this could be very costly.